This invention relates generally to magnetic resonance imaging (MRI), and more particularly the invention relates to MRI using steady state free precession (SSFP) with banding artifact reduction.
Magnetic resonance imaging (MRI) is a non-destructive method for the analysis of materials and represents a new approach to medical imaging. It is generally non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
Magnetic resonance (MR) imaging is based on nuclear spins, which can be viewed as vectors in a three-dimensional space. During a MR experiment, each nuclear spin responds to four different effects—precession about the main magnetic field, nutation about an axis perpendicular to the main field, and both transverse and longitudinal relaxation. In steady-state MR experiments, a combination of these effects occurs periodically.
Refocused steady-state free precession (SSFP) sequences have recently gained popularity in magnetic resonance imaging, due to improved gradient hardware. SSFP imaging provides high signal and good contrast in short scan times. However, in regions of high local magnetic field variations, SSFP images often suffer from characteristic bands of signal loss, or “banding artifact”.
Several multiple acquisition methods to reduce the banding artifact have been described in the prior art. In these schemes, multiple sets of images are acquired each with a different RF phase increment from one sequence repetition to the next (RF phase cycling). The images are combined in a way to effectively reduce the dark band artifact. However, the extra cost in scan time (usually up to a factor of 4) is limiting.